Bring a medium saucepan of water to a boil. Add the lentils and onion and season with salt and pepper. Reduce the heat to low, cover and simmer until the lentils are tender, about 15 minutes. Drain; discard the onion. Transfer the lentils to a medium bowl and let cool.

Cook the asparagus tips and stems in a medium saucepan of boiling salted water until barely tender, about 4 minutes. Drain and rinse in cold water, then drain again and pat dry. Add the asparagus to the lentils. Add the fennel, celery, tomatoes, jalapeños, cilantro, parsley, lemon juice, olive oil and soy sauce to the salad, season with salt and pepper and toss to mix.

Arrange the endive spears on plates. Mound the lettuce in the center and top with the asparagus-lentil salad.